Energy conservation in metropolitan mass rapid transit systems can same electric energy, free generating capacity for other electric power needs, and produce substantial dollar savings. Thus, for example, in the New York City subway system, the efficient application of regenerative braking (i.e., recovery, storage and re-employment of kinetic energy possessed by a moving vehicle at the onset of braking) can lead to energy savings of 40%, which is equivalent to a savings of 40 million dollars a year.
One of the difficulties in implementing regenerative braking with conventional, electric-motor-propelled subway cars is that the recovery of the kinetic energy of a speeding train is limited by the natural receptivity of the system, i.e., the proximate availability of accelerating trains to accept the power generated by decelerating trains. An earlier study has shown ("Energy Conservation in the New York Subway System," Schulz, H. W., H. E. Meadows, G. A. Domoto et al., Sept. 30, 1976, Columbia University) that the natural receptivity of the New York subway system is only 40%. This factor alone would reduce the potential energy savings from 40% to 16%. In addition, the series-parallel configuration of the present New York City cars would impose additional inefficiencies (by virtue of voltage limitations) in returning regenerated power to the third rail.
The above limitations are eliminated if provision is made on each car or train to store the kinetic energy possessed by the train at the inception of braking in a form that could readily be utilized to accelerate the train after it has stopped at a station. One means of accomplishing this objective is to equip each car with one or two spinning flywheels which can store the regenerated energy mechanically. The technical efficiency of this means of energy storage has been demonstrated in revenue service by experimental flywheel cars operated by the New York City Transit Authority. However, the cost of retrofitting the existing cars with flywheels has been shown to be prohibitive. Even when considering the purchase of new cars, the flywheel car configuration is a very costly option both from the standpoint of capital cost and incremental maintenance charges.
The present invention provides a novel system of on-board energy storage which comprises a light-weight, low-cost hydrostatic transmission and a gas accumulator which has no moving parts.
Another propulsion system which has utility for the storage of kinetic energy during vehicle deceleration is disclosed in U.S. Pat. No. 3,903,696 of Carman. That prior art system generally discloses elements of a hydrostatic transmission and gas accumulator primarily for use in storing energy of a single unit vehicle, such as a trailer or tractor and the like. This system employs a conventional hydrostatic transmission and conventional cylindrical pressure vessel to serve as the gas accumulator. Both of these prior art devices contribute excessive weight to the vehicle which has precluded their commercial utilization because of the resultant high capital and operating costs.
It is a purpose of the present invention to provide a propulsion system of markedly reduced weight by uniquely combining a hydrostatic transmission of the variable displacement radial piston type with novel gas accumulator pressure vessels that serve as structural members of the vehicle, thus avoiding the introduction of redundant weight.